1. Field of the Invention
The present invention relates to an image forming apparatus having a buffering device on the upstream side of a sheet processing apparatus and on the downstream side of an image forming portion, and more particularly, to an image forming apparatus whose frame member has a buffering device and a sheet processing apparatus arranged therein.
2. Related Background Art
Recently, in image forming apparatuses, such as copying machines, printers, facsimile machines, and composite apparatuses composed thereof, there has been an improvement in performance in terms of image quality, number of prints per unit time, etc.; in particular, there has been a marked improvement in printing speed at the time of image formation. In view of this, the interval between the sheets on which image formation is to be performed is set as small as possible.
Conventionally, there exists an image forming apparatus, such as a copying machine, a printer, a facsimile machine, or a composite apparatus composed thereof in which a sheet on which an image has been formed on the obverse side (first side) thereof is conveyed again to the image forming portion to form an image thereon. In such an image forming apparatus, there is provided a re-conveying path for conveying a sheet with an image formed on the obverse side (first side) thereof to the image forming portion again to perform image formation on the reverse side (second side) thereof.
In such an image forming apparatus, a sheet accommodated in a sheet feeding cassette is fed by a sheet feeding apparatus, and then the sheet is sequentially conveyed to an image forming portion, a fixing portion, and a discharging portion. At that time, if image formation is to be performed on both sides of the sheet or in superimposition on one side thereof, the sheet is conveyed to a re-conveying path to convey it to the image forming portion again.
Further, as a conventional image forming system, there is available one having a sheet processing apparatus on the discharge side of the image forming apparatus main body for performing a processing such as binding sheets with images formed thereon; in the case of such an image forming system, in order to improve productivity of the image forming apparatus, there is provided, for example, in a sheet processing path through which sheets are conveyed to the sheet processing portion of the sheet processing apparatus, a buffering device, which is a buffering portion for causing sheets with images formed thereon to stay temporarily while the sheet processing portion is performing sheet processing.
By providing such a buffering device in the sheet processing apparatus, while the sheet processing portion is performing the processing operation, a plurality of sheets with images formed thereon are caused to stay temporarily by winding the sheets around a buffer roller provided in the buffering device, thus securing the requisite sheet processing time. In this buffering device, when the processing operation is completed, the plurality of wound sheets are discharged at a time toward the sheet processing portion (see JP 2000-153947 A).
Incidentally, in recent image forming apparatuses, the printing speed (the image forming speed) up to the discharge is set high, whereas the discharging speed when discharging and stacking sheets with images formed thereon is set relatively low in order to secure the requisite stacking alignment. For, if the sheets are discharged while maintaining the high printing speed, there is a possibility of the sheets going so far as to interfere with the stacking alignment. Thus, as in the case of securing the sheet processing time, in order to adjust the difference between the printing speed and the discharging speed, it is necessary to cause the sheets with images formed thereon to stay temporarily.
Further, in the conventional sheet processing apparatus, as the buffer roller of the buffering device, there is used a large roller having a circumferential length longer than the sheet length so that a plurality of sheets can be properly wound around it. However, when such a large roller is used, the size of the buffering device increases, with the result that the size of the sheet processing apparatus increases.
Further, in the sheet processing apparatus, there exist a plurality of sheet conveying paths, such as the sheet processing path, the buffering path provided in the buffering device and formed along the peripheral surface of the buffer roller, the discharging path for conveying sheets with images formed thereon to the discharging portion, and the re-conveying path. When a plurality of sheet conveying path exist in the apparatus main body, not only a rather complicated construction but also an increase in the size of the sheet processing apparatus is involved.